Apparatus for high-frequency communication



Nov. 26, 1 929. H. B. BABSON ET AL. 36, 70

AYPARATUS FOR HIGH FREQUENCY COMMUNICATION Filed Jan. 15, 19%

L INVENTORS! k1 HENRY B. BABSON ac GEORGE WARICK ANKERSEN,

Patented Nov. 26, 1929 UNFTED STATES PATENT OFFICE i OF CHICAGO, ILLINOIS APPARATUS FOR HIGH-FREQUENCY COMMUNICATION Application filed January 13, 1926. Serial No. 80,904.

This invention relates to high frequency communicating systems, and more particularly to a method and apparatus for increasing the selectivity and efficiency of a radio .5 receiving system. The principal objeet of the invention is to provide a system in whlch the signal energy of the received high frequency waves effective in a wave responsive device may be maintained at the highest possible value, while at the same time the energy losses in the responsive device incident to the reception and utilization of the high frequency waves are prevented from impairmg selectivity or the ability to discriminate be- 1 tween received impulses of slightly differing frequencies. The invention is of particular utility in its application to radio communleating systems employing thermlonic valves of the three electrode type, and especially 111 the portions of such systems where it is desirable to preserve a high degree of selectivity while operating the thermionic valve under conditions which produce conductance and capacity losses in its grid circuit.

As is well-known, it is customary in radio receiving systems employing thermionic valves or audions to secure mammum response of the valves to the wave frequency of the desired station and minimum response to the wave frequencies of other undesired stations by associating tuning circuits with the valves. In such a tuning circuit the sharper the resonance curve the greater is the peak voltage efiective upon the input circuit of the audion and the greater is the ability to discriminate between the frequency for which the tuning circuit is adjusted and other frequencies differing slightly therefrom. It is also well-known that the resonance curve 1s broadened and flattened, with a consequent reduction of the peak voltage and loss of selectivity by the introduction of resistance into the resonant circuit. Such resistance maybe present in the resonant circuit itself, or it may be introduced through the medium of operating circuits associated with the resonant circuit. Where the grid circuit of a thermionic valve or audion, as is usually the case, is directly in shunt of the inductance and 0 capacity elements of the associated grid tuning resonant circuit these energy dissipating characteristics of the grid circuit have the effect of introducing resistance losses in the resonant circuit that materially reduce its peak voltage and diminish its selectivity. This has the result of reducing the effective energy in the grid circuit derived from the high frequency wave in favor of which it is desired to discriminate, and increasing the grid circuit energy derived from other undesired frequencies.

In accordance with the principles of our invention we provide a system in which the grid circuit is so organized and related to the associated tuning circuit that it is capa- 05 1c of abstracting a maximum of energy from the tuning or resonant circuit without materially affecting the resonant characteristics of the tuning circuit. This is done by separating the grid circuit from the tuning cireuit, and giving the grid circuit such characteristics and such relationship with the tuning circuit as best adapted for building up maximum voltage from the tuning circuit flux, without reacting upon the tuning circuit to impair its selectivity. By this effective separation of the grid circuit and the tuning circuit the desired characteristics may more conveniently be given to each, and by regulatingthe coupling between the two circuits 30 greater selectivity in the tuning circuit and more efiective energy utilization in the grid circuit may both be attained. The manner in which these and other desirable results are secured will be set forth hereinafter.

One of the forms in whichthe principles of our invention may advantageously be embodied is illustrated in the accompanying drawing, which diagrammatically represents a radio receiving system employing a thermionic valve or audion as a detector.

Referring to the drawing, the energy collecting conductor, which is illustrated in this embodiment as an antenna 10, is connected with ground through the medium of an antenna coupling coil 11. The coupling coil 11 is inductively coupled with the inductance element 12 of a resonant or tuning circuit which comprises the inductance element or tuning coil 12 and the variable tuning 0011- denser 14. A grid or input circuit coupling coil 13 is arranged in loose inductive coupling relation with the inductance element or coil 12 of the tuning circuit. The grid coil 13 has one of its terminals connected with the grid of the detector tube 15 through the usual grid condenser 17 and grid leak 18, while its other terminal is connected to the positive terminal of the A battery and the corresponding terminal of the filament of detector tube 15. The current flow in the filament of detector tube 15 is controlled by the rheostat 16. The plate circuit of the detector tube is connected with the positive terminal of the B battery by way of the telephone receiver or head set 29, and the by-pass condenser 20 is included in a branch circuit in the usual way.

We shall now particularly describe the characteristic features of our invention and the manner in which the invention cooperates in a radio receiving circuit of the type which has been chosen to illustrate one embodiment of the invention. In the first place particular mention will be made of the preferred characteristics of the coils through the medium of which the antenna, tuning circuit and grid circuit are brought into operative relation with each other. The antenna coil 11, in the matter of the number of turns employed and the location of the coil with respect to the coil 12 of the tuning circuit, depends upon the antenna which is used and the coupling that is desired. Ordinarily the antenna coupling coil is wound with a relatively small number of turns and is placed fairly close to the tuning coil 12 so that there may be a relatively uniform flux linkage between the two coils throughout the range of frequencies that it is desired to receive.

The tuning coil may preferably be so wound and related to the associated variable condenser of the tuning circuit as to constitute thetwo, as nearly as possible, a purely resonant circuit with a minimum of radio frequency resistance. The coil or inductance element 12 of the tuning circuit consists of a number of turns of wire, preferably of low resistance, which turns, if desired, may be spaced apart in order to reduce the radio frequency resistance.

The grid coil 13 is preferably wound with a comparatively large number of turns of relatively fine wire, the turns preferably being of relatively'small diameter. The wind ing preferably is such as to givethe grid coil as little distributed capacity as possible, and as much inductance as possible short of a value which, coacting with the distributed capacity of the winding, will carry the natural period of the coil into the frequency band that it is desired to receive. In other words, the grid coil preferably should not have a fundamental wave length that is equal to or greater than the shortest wave length to be received; and subject to this limitation the capacity of the grid coil should be as small and the inductance of the coilas large as possible. By making the convolutions of the grid coil of relatively small diameter the distributed capacity for a given inductance is made less; and by using relatively fine wire and thus minimizing the amount of metal in the field of the tuning coil the tendency to energy losses in the tuning coil due to eddy current action is reduced. The grid coil is so placed with reference to the tuning coil that the coupling between them is loose, so that energy is transferred from the tuning circuit to the grid circuit without the resonant characteristics of the tuning circuit being substantially affected by the resistance loss characteristics of the grid circuit, which losses on account of the conductance and capacity relations obtaining in the grid circuit may be relatively large.

In one set of antenna, tuning and grid coils that wehave tried and found to operate satisfactorily the antenna coilhad ten turns of #22 wire of three and one-quarter inches diameter, the tuning coil was wound with twenty-five turns of #17 wire, the turns in this particular coil being slightly spaced apart and the diameter of the coil being four inches, and the grid coil was wound with one hundred turns of #40 wire and with a diameter of two inches. The antenna coil was placed coaxially with the tuning coil and just inside one end of the coil; and the grid coil was also arranged coaxially and separated by a distance of approximately two inches from the other end of the tuning coil. It is to be understood that the specific set of values et which was found to give satisfactory results, andthat obviously other sets of values may readily be determined and employed by those skilled in the art.

It may be seen that'with an organization of the grid circuit and the tuning circuit and a relationship between them such as that de scribed above it becomes possible readily to control the characteristics of each circuit and the reactions of the circuits upon each other. By the separation of the grid circuit from direct connection with the resonant tuning circuit, and the substitution of a grid coil in controllable coupling relation with the tuning circuit coil, the grid circuit may be made an untuned circuit, or a circuit having a fundamental frequency. in the matter of its coupling coil, which is outside of the frequency band that it is desired to receive. At the same time this separation of the untuned grid circuit from the resonant tuning circuit makes it possiblereadily to adjust the coupling relation between the two circuits to just the degree which will permit the maximum transfer of energy and building up of voltage in the grid circuit without transferring 'iven above are merely representative of one the resistance load of the grid circuit to the tuning circuit to a degree that will materially impair the resonant and selective qualities of the tuning circuit. In this way it becomes easy to secure maximum volt-ages in the grid circuit and at the same time maintain a high degree of selectivity in the system. By employing the characteristic features of this invention it is unnecessary to introduce a large amount of inductance into the resonant circuit to secure'a high voltage in the grid circuit, for the reason that it is the power of the tuning circuit converted into magnetic flux, rather than the potential across the resonant circuit inductance, that acts through the medium of the coupled inductance coil of the grid circuit to build up the grid circuit potential.

It will, of course, be apparent to those skilled in the art that the coupling between the grid circuit and the tuning circuit may be capacity coupling rather than the mutual inductance coupling which is illustrated as being employed in the particular embodi- Q 3 ment of the invention that has been illustrated and described.

Having thus described our invention, what we claim and desire to secure by Letters Patent is:

1. In a high frequency communicating system, the combination of a high loss work coil comprising a large number of turns of relatively fine wire, a resonant tuning circuit comprising a low loss coil having relatively few turns of relatively coarse wire and mounted in co-axial alignment and loose inductive relationship with the said work coil, and means for energizing said tuning circuit comprising an input coil comprising relatively few turns of relatively coarse wire and having a close inductive coupling with the coil of said tuning circuit and a negligible inductive coupling with the work coil.

2. In a high frequency communicating systern, the combination of a high loss work coil, a resonant tuning circuit comprising a low loss coil mounted in co-aXial alignment and loose inductive relationship with said work coil, and means for energizing said tuning circuit comprising an input coil having a close inductive coupling with the coil of said tuning circuit and having a relatively negligible coupling with the work coil.

In witness whereof, We have hereunto subscribed our names.

HENRY B. BABSON.

GEORGE WARICK ANKERSEN. 

